Compromised ITAM-based platelet receptor function in a patient with immune thrombocytopenic purpura

Background: Receptors on platelets that contain immunoreceptor tyrosine-based activation motifs (ITAMs) include collagen receptor glycoprotein (GP) VI, and Fc gamma RIIa, a low affinity receptor for immunoglobulin (Ig) G. Objectives: We examined the function of GPVI and Fc gamma RIIa in a patient diagnosed with immune thrombocytopenic purpura (ITP) who had unexplained pathological bruising despite normalization of the platelet count with treatment. Methods and Results: Patient platelets aggregated normally in response to ADP, arachadonic acid and epinephrine, but not to GPVI agonists, collagen or collagen-related peptide, or to Fc gamma RII-activating monoclonal antibody (mAb) 8.26, suggesting ITAM receptor dysfunction. Plasma contained an anti-GPVI antibody by MAIPA and aggregated normal platelets. Aggregating activity was partially (similar to 60%) blocked by Fc gamma RIIa-blocking antibody, IV.3, and completely blocked by soluble GPVI ectodomain. Full-length GPVI on the patient platelet surface was reduced to similar to 10% of normal levels, and a similar to 10-kDa GPVI cytoplasmic tail remnant and cleaved Fc gamma RIIa were detectable by western blot, indicating platelet receptor proteolysis. Plasma from the patient contained similar to 150 ng mL(-1) soluble GPVI by ELISA (normal plasma, similar to 15 ng mL(-1)) and IgG purified from patient plasma caused Fc gamma RIIa-mediated, EDTA-sensitive cleavage of both GPVI and Fc gamma RIIa on normal platelets. Conclusions: In ITP patients, platelet autoantibodies can curtail platelet receptor function. Platelet ITAM receptor dysfunction may contribute to the increased bleeding phenotype observed in some patients with ITP.